The invention relates to devices for administering injectable products, in particular the injection of medically or cosmetically effective products.
Devices which the invention also concerns are known as injection pens. Generally, an injection pen comprises an elongated, hollow cylindrical housing in which a container, having the shape of an ampoule, filled with the product to be administered, is received. An injection needle is attached to the container and when so attached is generally axially aligned with or parallel to the central longitudinal axis of the housing, extending generally from one end of the housing. A drive unit for a piston within the container is also arranged in the housing. The piston is advanced in forward direction within the container under the effect of a driven member of the drive unit, thus causing a predetermined product dose to be dispensed. For advancing the piston, the driven member itself is advanced in the forward direction (i.e., toward the needle) in relation to the housing, either manually or by the drive force of a drive element of the drive unit. In the latter case, energy is stored in the drive unit, which energy is converted into a drive force driving the driven member forward upon the drive unit being actuated.
In the course of advancement of the driven member, the stored energy or at least part of the same is consumed. The drive force exerted by the drive element on the driven member is consequently reduced while the driven member is being advanced. Typically towards the end of the injection or dispensing cycle, the advancing or forward speed of the driven member decreases and, therefore, the dispensing rate will decrease, i.e., dispensing is not uniformly distributed over the total injection or dispensing cycle. Other interferences or counter forces may be caused by irregularities in the internal diameter of the ampoule, resulting in the wall frictional forces affecting the piston, causing non-constant piston movement over the stroke of the piston and making uniform dispensing or injection difficult.